The procedures for storing, transmitting or processing digitized sound signals, used increasingly these days, come up against the problem of the ever more sizeable throughput required depending on the applications envisaged.
Very elaborate techniques, such as the coding of these signals by frequency transform, such as the modified discrete cosine transform, MDCT, are used to reduce the resulting data throughput, whilst substantially preserving the quality of the original signal. Thus, as has been represented illustratively in FIGS. 1a and 1b, the coding by transform proceeds by partitioning the time signal, succession of samples x(n), into blocks comprising N samples. Over each of the abovementioned blocks, after applying an amplitude--time weighting window, such as the window-of block m over the samples 0 to N-1 as represented in FIG. 1b, these windows being repeated with an overlap over N/2 samples, a time/frequency transform is applied to the block of samples thus weighted, to obtain a set of coefficients y(k) which are then coded and then transmitted with the throughput reduction sought, as indicated in FIG. 1a.
On decoding, an inverse transformation is applied to the received coefficients y'(k), this permitting regeneration of the original samples x'(n) in real time, to within coding-decoding processing time and transmission time. It will however be noted that several types of frequency transform may be used, the MDCT transform making it possible however to obtain the largest throughput reduction. For a more detailed description of this type of processing, reference can be made for example to the publications by Johnston "Transfer Coding of Audio Signals Using Perceptual Noise Criteria" IEEE Journal on selected Areas in Communications, Vol. 6, No. 2, February 1988, pp. 314-323, and by Princen and Bradley "Adaptive Transform Coding incorporating Time Domain Aliasing Cancellation" Speech communication, December 1987.
In frequency transform coding systems, the coding noise resulting from the digitizing of the coefficients is distributed uniformly throughout the duration of the sample block.
When this block contains a non-stationarity, such as a sudden excitation, like the one represented in FIG. 1b, the frequency spectrum of the signal, that is to say the distribution, as a function of frequency f, of the amplitude of the coefficients y(k) obtained after. frequency transform, is almost flat, as shown in FIGS. 1C2 and 1C3.
Furthermore, the abovementioned coding algorithms generally carry out a spectral shaping of the noise (cf. abovementioned publication by Johnston) and, even in regard to the block containing a transition, such as mentioned earlier, the spectrum of the noise is almost flat and constant in level throughout-the duration of the block.
Accordingly, for the part preceding the excitation, the spectrum of the noise is clearly greater than that of the signal. This is reflected in the time domain, as represented in FIGS. 1d and 1e relating to an original signal x(n) from a vibraphone excitation sampled at 32 KHz, and to a corresponding signal decoded and then subjected to an inverse transform x'(n) at 64 KBITS/Sec, respectively, by the appearance of a very large degradation called pre-echo.
Furthermore, when the coding system uses the MDCT transform, the two blocks preceding the transition are, owing to the overlap of N/2 samples between successive windows, affected by the pre-echo phenomenon.
Various processing methods have been proposed in order to reduce or eliminate the pre-echo phenomenon.
Among these can be cited the method advocated by Schroeder, described in the article entitled "Stereo Coding with CD quality" International Conference on Consumer Electronics, Chicago 1987 and by Sugiyama, described in the article "Adaptive Transform Coding with an adaptive Block Size", Proceedings of ICASSP 90, Albuquerque pp 1093-1096. In the abovementioned method, a scale factor is applied to the sample block before transformation, so as to reduce the difference in level before and after the transition generated by the excitation. The application of the inverse scale factors to the decoding makes it possible to reduce the noise level in the low energy zones and hence the amplitude, whence the corresponding reduction in the pre-echo phenomenon. However, such a method cannot validly be used when the frequency transform is an MDCT transform, it not being possible to reconstruct the signal perfectly, in the absence of coding, owing to the very presence of the scale factors.